Automatic grinding and polishing machines for gem stones are known. These generally include a plate for holding a plurality of dops in a horizontal row, means for rotating the dops around their respective longitudinal axes, and means for pivoting the plate such that gem stones mounted on the dops move in and out of engagement with a grinding or polishing wheel or belt in accordance with a pre-determined sequence.
These machines are much faster than manual grinding and polishing. On the other hand, no individual treatment of stones is possible. Thus, in order for use of the machine to be economically feasible, stones of approximately the same size must be worked together (so as to avoid removing too much material from a relatively large stone or insufficient working of a relatively small stone). In addition, the stones must be accurately centered on the dops to permit efficient working of each stone. Finally, all the stones must be disposed in the machine with their tables in a sensible identical plane.
At present, centering and affixing the stones on the dops are generally performed manually, depending exclusively on the worker's skill and eye. This leads to a certain amount of error, resulting in stones which either must be rejected or reworked to a smaller size. Thus, use of such machines today is economical only where the stones to be worked have relatively little value so that human errors are relatively inexpensive.
There is known a device called a comparator which comprises an illuminated screen on which a grid is defined. A stone may be held in front of the comparator and viewed from a direction perpendicular to the axis of centering to permit visual centering by an operator. Furthermore, in order to examine a stone relative to a plurality of shapes or sizes, a different reference image must be manually affixed to the comparator for each comparison.